This invention relates to a rotary throttle type carburetor suitable for use with a small internal combustion engine, for powering portable implements such as hand held chain saws, weed trimmers, brush cutters and the like, more particularly to a fuel regulating mechanism for such a rotary throttle type carburetor.
Rotary throttle type carburetors are currently used to provide the combustion fuel requirements for a wide range of two-stroke-cycle and four-stroke-cycle engines, including hand held engines, such as engines for chain saws and weed trimmers. Typically these carburetors are diaphragm type utilizing a fuel-metering diaphragm operative to control the delivery of fuel from the carburetor regardless of its orientation. There is an increasing trend to provide a so-called xe2x80x9cmini-four-strokexe2x80x9d type small engine in order to achieve better fuel economy and reduced exhaust gas air pollutants as compared to a comparable two-stroke cycle engine. However, the very minute quantity of fuel required to power a mini-four-stroke at idle speed in turn requires that the idle mixture needle be set to establish a very tiny overall idle outlet opening in the fuel jet port of the fuel supply pipe. This in turn can lead to problems of sensitivity to needle tip axial movement as well as clogging from debris in the fuel.
As is well understood in the art, a rotary throttle type carburetor typically comprises a cylindrical throttle valve having a throttle hole disposed in the air intake passage of the carburetor body, and the quantity of combustion air intake to the engine is controlled by rotation of the throttle valve. The quantity of fuel delivered to the engine is controlled by the relative position of a needle attached to the throttle valve that is raised and lowered by a cam that rotates with the throttle valve so that the tip of the mixture needle moves along a fuel jet side port of a fuel supply pipe to vary the open area of the fuel jet port.
There are various known methods for regulating the low speed or idle speed fuel delivery of such rotary valve carburetors. One such method and mechanism is disclosed in Japanese Patent Application Publication No. 110847/1983 and in corresponding German Patent DE 3247603 A1 (1983), FIG. 2 of which is also shown as prior art in FIG. 5 of U.S. Pat. No. 5,709,822 and described therein at column 1, lines 47-60, as follows:
A valve type carburetor disclosed in Japanese Patent Laid-Open No. 110847/1983 is known in which, as shown in FIG. 5, in order to change a flow of air with respect to a fuel pipe 16 which projects toward a throttle hole 17b of a rotary throttle valve 17, that is, in order to change a suction negative pressure exerting on a fuel jet port 16a at an idle position of the throttle valve 17, a through-hole 17c opening to an inlet of an air intake passage 44 is provided in a wall portion of the throttle 17b of the throttle valve 17. In this proposal, the inside diameter of the through-hole 17c is selected according to the specification of the engine. Therefore, the fuel quantity at the idle position is fixed to a predetermined value and cannot be freely adjusted.
In the system and mechanism of the aforementioned U.S. Pat. No. 5,709,822, and as best seen in FIG. 3 thereof, an air bleed bypass passage 41 is provided for communicating the main throttle hole or throttle bore 17b of the throttle valve 17 with the carburetor intake passage 44 of the carburetor body upstream of the throttle valve. An air quantity regulating needle valve 43 is provided in this bleed passage for adjusting the quantity of bypass air admitted to the rotary throttle valve throttle hole 17b. 
In order not to exceed the permitted maximum adverse emissions limit of EPA and/or CARB exhaust regulations, the air/fuel (A/F) mixture is set at the factory by permanently adjusting the conventional fuel regulating needle 15 so that at idle throttle setting the size of the fuel jet orifice 16a is made small enough to establish the maximum fuel delivery at engine idle speed that is permitted in terms of the applicable exhaust gas regulations. This is done while the air bleed bypass regulating needle valve 43 is screwed in to completely close bypass or block passage 41. Then an anti-tamper closing member (i.e., ball 62) is forced into the mixture needle mounting hole 47a and sealed off (as by adhesive 61) so that the fuel regulating needle cannot again be regulated from outside.
However the operator can still regulate (i.e., lean out), if desired, the fuel quantity in the engine idle operating range. The quantity of idle bypass air flowing through the bypass air bleed passage 41 for bypass communicating the throttle hole 17b of the throttle valve 17 with air intake passage 44 upstream of the throttle valve is regulated by adjusting the air quantity regulating needle valve 43. If the quantity of air flow through the air passage bypass 41 is thus increased, the A/F mixture becomes leaner, and if this bypass air quantity is decreased, the mixture becomes richer. However, since the maximum concentration of the fuel in the A/F mixture at throttle idle setting has been preset, the idle A/F mixture will not exceed the permitted maximum value of the exhaust gas regulations. That is, even if the air quantity regulating needle valve 43 is fully opened, and even if the air quantity regulating needle valve 43 is removed, the bypass air quantity merely becomes maximum, thus the concentration of the mixture does not become rich because the maximum rate of fuel delivery is independently controlled and has already been preset by the aforementioned factory pre-adjustment of the fuel regulating needle.
Although the adjustment feature of ""822 patent air quantity regulating needle valve in the idle bypass passage is a desirable feature in many applications, neither it nor the aforementioned Japanese Laid-Open patent cited therein as prior art to ""822 solves the problems of regulating needle sensitivity and clogging of the idle output opening as so established by factory adjustment of the conventional fuel regulating needle.
Moreover, other problems associated with adapting a rotary valve type carburetor to the characteristics of a mini-four-stroke engine are neither recognized nor addressed by these aforementioned prior art documents. For example, there is no way the mechanism can be adjusted to provide a simple enrichment starting system to assist cold start of such an engine (that does not require the addition and use of the current standard choke system for this purpose,) and without affecting wide open throttle (W.O.T.) performance. Also, there is no recognition of nor provision for solving the problem of adjusting the fuel quantity versus engine speed curve produced by the regulated A/F mixture in the range of throttle settings between idle and full throttle to better match the performance requirements for acceleration of the engine in the part throttle range. These problems are particularly acute in small mini-four-stroke engines which are highly sensitive to rich and undesired fuel and air mixture provided to the engine.
Accordingly, among the objects of the present invention are to provide an improved fuel regulating mechanism for a rotary throttle valve type carburetor, and improved method of operating the same, that overcomes the aforementioned problems, particularly those associated with providing such a carburetor for a mini-four-stroke engine, that provides an improved method of controlling the amount of vacuum or negative pressure exerted on the idle fuel outlet orifice at idle speed setting of the carburetor without significantly reducing the throttle valve opening, that provides a low cost and easy to operate improved starting system for such an engine, as well as other types of engines utilizing rotary throttle valve carburetors, and enables the permanent factory adjustment of the fuel regulating needle to be set xe2x80x9chigherxe2x80x9d to establish a larger overall idle outlet opening, and hence one that is much less sensitive to needle tip axial movement and the problems of clogging of the idle outlet opening from debris in the fuel flow, that can be factory set in a secure manner to observe exhaust gas emissions regulations and also adjustable by design and/or in operation to improve engine performance in idle, part throttle and high-speed operating modes of the engine, that can be used as a simple enrichment starting system in that, unlike current standard choke systems, does not affect W.O.T. operation, and that utilizes an improved air bleed passage that even if inadvertently left closed will still enable the engine to idle satisfactorily, albeit somewhat rich, and in any event will perform as normal at W.O.T.
Another object of the invention is to provide an improved fuel regulating method and mechanism of the aforementioned character for a rotary throttle (barrel-type) carburetor that enables the air/fuel (A/F) mixture to be factory calibrated to adjust the acceleration ramp or curve of fuel flow versus engine speed so that part throttle operation can be enriched as desired to meet the characteristics of a given engine without requiring the re-installation of a throttle cam plate having a different cam surface or ramp contour selected from an inventory of such cam plates heretofore provided to attempt to satisfy this carburetor-to-engine calibration requirement.
A further object is to provide an improved fuel regulating mechanism and method of the aforementioned character that is capable of achieving the aforementioned objects and yet is of relatively simple design, economical in manufacture and assembly, rugged, reliable, durable and has a long useful life in service.
In general, and by way of summary description and not by way of limitation, the invention accomplishes one or more of then foregoing objects by providing an improved method of and mechanism for regulating fuel feed from a carburetor to an associated engine. The carburetor is of the aforementioned rotary throttle valve type with a throttle hole disposed in an air intake passage of the carburetor body. Rotational movement of the throttle valve varies the opening area of the throttle hole exposed to the carburetor intake passage for controlling the air flow therethrough. The quantity of fuel released from a fuel jet port of a fuel supply pipe secured to the carburetor body is controlled by the relative position to such jet port of a fuel regulating needle attached to the throttle valve for axial movement therewith. Adjustment of needle regulation of the fuel jet port cannot be made from outside of the carburetor after an idle speed fuel quantity has been set and then the permanent fitment of a closing member.
The carburetor further also has a bypass air passage for variably communicating the throttle valve hole at an upstream portion thereof with a bypass air source, such as ambient atmosphere or the upstream intake air in the carburetor, in bypass relation to the opening area of the throttle hole exposed via a bypass air passage outlet operable at engine idle setting of the throttle valve. The bypass air passage outlet is closed by movement of the throttle valve out of idle setting toward high speed and/or maximum power setting.
Preferably, a bypass air quantity regulating valve is provided in the bypass air passage to variably adjust the quantity of air flowing in the bypass air passage to the throttle hole. At initial carburetor-to-engine set-up and calibration, the bypass air regulating valve is maintained open while the engine is running at idle speed, such as by operating the air valve to a given open setting.
Then the fuel-regulating needle is adjusted to provide the maximum fuel to air (F/A) mixture ratio permitted by applicable engine exhaust air quality regulations. Next, the fuel needle adjustment is permanently set by non-removably fitting the closing member to prevent exterior access to an adjustment portion of the fuel needle.
Preferably thereafter, during subsequent end user operation of the engine, the bypass air regulating valve is closed only when preparing to crank the engine for starting to thereby provide an enriched fuel-to-air mixture for starting of the engine. When the engine is running under its own power the bypass air regulating valve is maintained open.
As an option, the bypass air regulating valve can be adjusted to vary the air flow regulating opening of the same from the given setting to thereby re-adjust the initial set-up idle F/A mixture to a different, leaner or richer, value for end user engine operation. The bypass air regulating valve also may be in the form of a solenoid-operated valve operably coupled to the engine control system such that the valve automatically is closed for engine start up and automatically opened when the engine begins to run under its own power. As a further option, the bypass air regulating solenoid valve has an adjustable end-limit open stop for adjusting its open setting to thereby increase or decrease the air flow regulating opening end limit of the same to re-adjust the initial set up F/A mixture to a different value for engine operation.
In one embodiment, the bypass air passage comprises a tubular conduit extending through a wall of the carburetor to an external connection with a bypass air regulating valve. The bypass air regulating valve may alternatively be (1) a movable flap valve for controllably opening and closing an open upstream inlet of the tubular conduit disposed externally of the carburetor, (2) a solenoid valve having an armature mounted in the tubular conduit with an armature plunger reciprocable therein and having a valve member at its distal end operable for opening and closing a valve port in a valve disk mounted in the tubular conduit, or (3) a normally closed thermal valve that is thermally responsive and operably coupled to the engine to sense and respond to engine operational heat of a given temperature to thereby open the bypass valve.
The bypass passageway, also alternatively, may take the form of a bypass inlet branch passage and a bypass outlet branch passage in the carburetor body, with the inlet opening of the inlet branch passage being located upstream of the throttle valve and the outlet of the outlet branch passage being located for communication with the throttle valve throttle hole in the idle position thereof. For ease of manufacture and calibration, the branch passages are preferably communicated with one another via a chamber in the carburetor exterior surface that is closed by a Welch plug. Preferably, the branch passages are drilled parallel to one another and generally perpendicular to the axis of the carburetor air intake passage.
In this embodiment a solenoid valve may be provided with a needle valve armature having a needle nose at its distal end cooperative with a valve seat formed in one of the branch passages. Preferably this valve seat is at the end of the bypass inlet branch passage entering the Welch plug chamber.
Preferably, and in lieu of changing throttle cam plates from an inventory having different ramp angles, the outlet of the bypass outlet branch passage is located relative to travel of the upstream control edge of the throttle valve throttle hole so as to modulate by design the fuel-to-air mixture ratio curve of fuel flow versus engine speed during part-throttle travel of the control edge past the outlet of the bypass outlet branch passage.